The present invention relates to a process for the separation and recovery of ruthenium from substrates of catalysts and/or electrodes and, more particularly, to a process for the recovery of ruthenium in a form in which it can be directly recycled for the preparation of fresh catalyst and/or electrodes.
Various salts of ruthenium have long been known to have utility in a wide variety of catalytic applications. Most commonly, a suitable salt of ruthenium, either alone or with salts of one or more additional metals, was deposited on a catalyst support such as alumina, titania, zinconia, silica, or silica alumina.
More recently, ruthenium has found significant commercial application as a coating for electrodes, particularly so-called dimensionally stable anodes. In such applications, the ruthenium may be deposited by electroplating or by other deposition means, either as the metal or in the form of a suitable salt, on an electrode substrate such as, for example, titanium, tantalum, or the like. Such electrodes and the method of their preparation are described, for example, in U.S. Pat. No. 3,096,272, U.S. Pat. No. 3,236,756, U.S. Pat. No. 3,265,526, and U.S. Pat. No. 3,711,385 to mention but a few.
The wide variety of advantages obtained by the use of such electrodes almost immediately established them as commercially significant. Since their first introduction, these electrodes have not only found almost universal acceptance in their original commercial applications, but are continually being modified for an ever-widening variety of additional commercial applications. This, in turn, made it more and more economically desirable to recover the ruthenium from used or defective anodes.
Historically, the treatment and recovery of precious metals and their salts, at least at the commercial level, has been largely a matter of proprietary information rigidly maintained as confidential of which a little, if any, has been published in the patent or technical literature. U.S. Pat. No. 3,761,313 (issued Sept. 25, 1973) discloses and claims a method of stripping the coating from such electrodes by immersing the electrode in an aqueous solution of hydrofluoric acid and at least one other strong mineral acid (other than nitric acid). In spite of this, the accepted commercial procedure for removal of the noble metal coating from such electrodes continues to be a mechanical abrasive procedure (such as, for example, the so-called wheel-abrator process), even though this contaminates the precious metal residue with large quantities of iron filings, sand, and hydrochloric acid. In fact, in commercial application, such a mechanical abrasive procedure provides residues containing only about one to ten percent (on a dry basis) precious metal.